Abstract
Upon depletion of combined nitrogen, about 10% of vegetative cells within the filaments of Anabaena sp. PCC 7120 differentiate terminally into heterocysts. The heterocyst has been studied as a model system of prokaryotic cell differentiation, with major focus on signal transduction and pattern formation. However, little is known about how early differentiating heterocysts are selected after the induction by nitrogen deprivation. To address this question, the expression of the hetR gene was visualized using GFP and followed by a microscopy-based system. The intensity of fluorescence of GFP in the transformants of hetR-gfp increased markedly in the future heterocysts at the fourth hour with respect to other cells. We also noted that the growing filaments consisted of clusters of four consecutive cells that we call quartets. High frequency of initial heterocysts originated from either of the two outer cells of quartets at the start of nitrogen deprivation. These results suggest that the future heterocysts are loosely selected at early times after the start of nitrogen deprivation, before the commitment.
